Bitumen is a heavy type of crude oil that is often found in naturally occurring geological materials such as tar sands, black shale, coal formations, and weathered hydrocarbon formations contained in sandstones and carbonates. Some bitumen may be described as flammable brown or black mixtures or tarlike hydrocarbons derived naturally or by distillation from petroleum. Bitumen can be in the form of anywhere from a viscous oil to a brittle solid, including asphalt, tars, and natural mineral waxes. Substances containing bitumen may be referred to as bituminous, e.g., bituminous coal, bituminous tar, or bituminous pitch. At room temperature, the flowability of some bitumen is much like cold molasses. Bitumen may be processed to yield oil and other commercially useful products, primarily by cracking the bitumen into lighter hydrocarbon material.
As noted above, tar sands represent one of the well known sources of bitumen. Tar sands typically include bitumen, water, and mineral solids. The mineral solids can include inorganic solids such as coal, sand, and clay. Tar sand deposits can be found in many parts of the world, including North America. One of the largest North American tar sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the tar sands formation can be found at the surface, although it may be buried two thousand feet below the surface overburden or more.
Tar sands deposits can be measured in barrels equivalent of oil. The Athabasca tar sands deposit has been estimated to contain the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global tar sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserves are estimated to be about 1.3 trillion barrels.
The bitumen content of some tar sands may vary from approximately 3 wt % to 21 wt %, with a typical content of approximately 12 wt %. Accordingly, an initial step in deriving oil and other commercially useful products from bitumen typically can require extracting the bitumen from the naturally occurring geological material. In the case of tar sands, this may include separating the bitumen from the mineral solids and other components of tar sands.
One conventional process for separating bitumen from mineral solids and other components of tar sands includes mixing the tar sands with hot water and, optionally, a process aid such as caustic soda (see, e.g., U.S. Pat. No. 1,791,797). Agitation of this mixture releases bitumen from the tar sands and allows air bubbles to attach to the released bitumen droplets. These air bubbles float to the top of the mixture and form a bitumen-enriched froth. The froth can include around 60% bitumen, 30% water, and 10% inorganic minerals. The bitumen-enriched froth is separated from the mixture, sometimes with the aid of a solvent, and further processed to isolate the bitumen product.
For example, the froth can be treated with an aliphatic (pentane-type) or an aromatic (naphtha-type) solvent to produce a clean bitumen product that can serve as a refinery upgrader feed stock. The bulk of the mineral solids can also be removed to form a tailings stream. The tailings stream can also include water, solvent, precipitated asphaltenes (in the case where the asphaltene is not soluble in the solvent used to separate the bitumen-enriched froth from the mixture), and some residual bitumen.
Tailings produced by the hot water process and/or the froth treatment process can pose several problems. Firstly, as noted above, the tailings produced by conventional methods can include solvents, precipitated asphaltenes, or residual bitumen. The bitumen and asphaltenes in a tailings stream represent unrecovered hydrocarbon that will not be processed into valuable commercial products. Accordingly, the conventional methods can result in a lower yield of hydrocarbon material, and consequently, diminished profit.
Additionally, the presence of bitumen and asphaltene in the tailings can complicate the disposal of the tailings because these materials present environmental risks. This can also be true for residual solvent included in the tailings that can be environmentally unfriendly.
The amount of tailings produced by conventional methods can also present chemical and physical problems. In some circumstances, the total volume of the tailings produced by the conventional methods may be more than the volume of mined tar sands, which means that not all of the tailings can be returned to the mined area.
The physical characteristics of the tailings can also present problems. The conventional methods sometimes utilize water and caustic as part of the process. This can result in the activation and swelling of certain clay components of a tailings stream. Accordingly, the tailings can have a sludge-like consistency that may last indefinitely. The sludge-like consistency means that the tailings are not stackable, thereby limiting the manner in which to dispose of the tailings. Often the only disposal option is to deposit the tailings in a tailings pond located outside of the mine area. These ponds can be costly to build and maintain and can be damaging to the local environment, including the local water supply. Furthermore, ponds can be damaging to the local wildlife population, such as birds, which can be caught in the oil and solvent laden tailings produced by hot-water extraction processes.